[PATCH] Fix duplicate errmsg in ALTER TABLE SPLIT PARTITION

Hi hackers,

The ereport() in transformPartitionCmdForSplit() that fires when
splitting a non-default partition while a default partition already
exists has two errmsg() calls:

ereport(ERROR,
errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
errmsg("can not split non-DEFAULT partition \"%s\"", ...),
errmsg("new partition cannot be DEFAULT because ..."),
parser_errposition(...));

The second one should had been errdetail()

The attached patch changes the second errmsg() to errdetail().

Also, I think "can not" can be replaced with "cannot" for consistency
throughout? Havent added is as part of this patch though. Thoughts?

Regards,
Ayush